1. Field of the Invention
The present invention relates to radiolocation systems, and more particularly to very short range pulsed microwave time-of-arrival systems. These systems can be used for wireless handwriting digitizing, a wireless computer mouse, machine automation, or virtual reality systems.
2. Description of Related Art
Precision short range location systems have employed ultrasound, optical, magnetic, inertial, and radio frequency (RF) technologies to locate a wireless moving object such as a writing pen for digitizing handwriting. While each technology has advantages, their disadvantages appear to have limited their widespread use.
Wireless pen location based on time-of-arrival (TOA) of ultrasound pulses can provide, at first glance, a highly accurate, low cost digitized handwriting system. However, the limitations to ultrasound are numerous and essentially insurmountable: (1) the speed of sound varies 10% over common outdoor temperature, greatly limiting accuracy, (2) ultrasonic noise from computer monitors, jangling keys, etc. reduce reliability, (3) propagation path blockage by a user""s arm or hand limit the choice of transducer locations to the top of the writing tablet, leading to severe geometric dilution of precision (DOP), (4) most ultrasonic transducers are highly directional, limiting coverage across a writing tablet, and (5) slow acoustic propagation velocity limits the pulse rate (due to range ambiguities), limiting response time to only accommodate slow handwriting.
Inertial location techniques use accelerometers mounted in a writing pen to provide position information by integration of acceleration signals. There is no absolute positional reference frame, forcing the user to locate writing position on a computer screen, like a mouse. Accuracy is limited by drift in the integrators needed to convert acceleration to position. In addition, low cost high accuracy accelerometers are not available, leaving inertial writing pen technology to specialty or high-end markets.
Magnetic writing pens employ a coarse X-Y wire grid under a writing tablet to detect oscillating magnetic fields from the tip of a writing pen, where the position between the nearest wires is interpolated from magnetic signal strength. Since magnetic field strength varies with the cube of range for close-in fields, interpolation can be difficult unless the grid spacing is kept small, requiring a large number of detectors or multiplexers. Movement of the pen in the Z-direction (above the tablet) would substantially alter the magnetic field gradient and the interpolation accuracy, preventing use with a thick pad of paper, and preventing 3-D operation.
Optical writing pens use light emitting diodes or lasers mounted in the pen and position sensitive detectors (PSD) mounted on a writing tablet or nearby fixture to locate the pen""s position. The PSD is formed with several photodiodes and lenses or with a strip-like photodiode and lens. In either case, proportional analog voltages are produced as a function of pen location, with an accuracy limited to perhaps 1% of full tablet range by photodiode gain differences, dark currents, image size and focus variations, background illumination, etc. Accuracy is also limited by the choice of PSD locations, which are confined to the top of the writing tablet where a user""s arm or hand will not block the light path, leading to severe geometric dilution of precision. Battery power limitations in the pen limit optical output power, and consequently, the signal-to-noise ratio at the photodiode receiver output, thereby limiting the ability to track fast handwriting. Optical devices are at a fundamental disadvantage to RF devices since photodetectors generally provide 30 to 60 dB lower output than a comparable microwave system with the same transmit drive power and range, and photodiodes cost more than a simple microwave antenna.
U.S. Pat. No. 5,589,838, xe2x80x9cShort Range Radio Location System,xe2x80x9d to McEwan, 1996, describes a time-of-arrival system employing a wireless xe2x80x9croverxe2x80x9d (i.e., moving object) that transmits short microwave pulses at an RF frequency of 2 GHz. An array of two or more receivers at known locations samples the RF environment to determine the relative arrival times of the transmitted pulses. The arrival times are converted to relative distances and the exact location of the rover is determined using simple geometric relations.
A key feature of the ""838 patent is the use of two-frequency timing. The receivers sample the RF environment at a first pulse repetition frequency PRFR that is slightly offset from a second frequency, the transmit pulse repetition frequency PRFT, by a frequency xcex94. For example, PRFT is 2,000,000 Hz and the receivers all sample with an offset frequency xcex94 of 100 Hz at a second frequency PRFR of 2,000,100 Hz (or 1,999,900 Hz). The offset frequency in the receiver, PRFR, causes the sample timing to slip 360xc2x0 in phase once each 1/xcex94 seconds. This phase slippage causes the sampled output of the receivers to produce an equivalent time replica of the 2 GHz RF pulses on a time scale related to the offset frequency xcex94, i.e., the phase slippage rate. There is a time expansion factor EF=PRFT/xcex94=20,000 for xcex94=100 Hz and PRFT=2 MHz, and the transmit pulse repetition interval PRIT=1/PRFT is expanded from xc2xd MHz=500 ns to a receiver output pulse repetition interval of F/2 MHz=10 ms. Thus, the arrival time signals from the receivers appear on a 20,000 times slower time scale, which makes precision detection and processing of the signals vastly simpler. This is important to many applications, such as handwriting digitizing, that require sub-picosecond accuracy.
A handwriting digitizing application of the techniques disclosed in the ""838 patent, using time-of-arrival receivers operating with two-frequency timing, is disclosed in U.S. Pat. No. 5,977,958, xe2x80x9cMethod and System for Digitizing Handwriting,xe2x80x9d to Baron, 1999.
According to the invention, a wireless transmitter mounted in a movable object, e.g., a handwriting pen, transmits microwave RF bursts at a PRF of 10 MHz (or any other selected frequency). Two or more receivers sample the microwave RF environment at the same 10 MHz rate. Upon receiving RF bursts from the transmitter, timing circuitry coupled to the receivers locks its 10 MHz PRF to the transmit 10 MHz PRF (with no offset xcex94). Once locked, there is no phase slippage between the transmit PRF and the receive PRF since they are locked to exactly the same frequency and phase. A swept timing circuit sweeps the sample timing to produce expanded time representations of the RF bursts that are suitable for precision processing into location fixes.
A key advantage to the use of RF bursts for handwriting digitization is the ability to locate receive antennas below a dielectric writing surface, including a thick pad of paper, where the RF propagation path cannot be blocked by a user""s arm or hand, or by the writing paper itself. Receive antennas can be located at the four corners of the tablet and at other locations below the writing surface, if desired, to essentially eliminate geometric dilution of precision. In addition, the pen may be accurately tracked while it is several inches (or more) above the tablet, allowing the pen to serve multiple functions as a writing instrument, a mouse and a joystick.
The present invention provides for operation with more than one transmitter housed within the moving object (or pen). This is accomplished by time-spacing the pulses from the additional transmitters. The use of two transmitters in a handwriting-digitizing pen provides pen tilt and inversion information, and facilitates 3-D location fixes with all the receiver antennas located below the writing surface.
A further advantage to the use of RF bursts is that short propagation times from a writing pen to receive antennas allow the use of very high pulse rates with a corresponding fast tracking rate of the pen and minimal dynamic distortion of written characters. Also, omni-directional antennas inside the writing pen make the pen rotationally independent and free the pen from transducer aperture blockage by the user""s hand (as may be the case with ultrasound or optical systems).
Yet another advantage of the present invention is the use of a very linear timing sweep, which provides inherently linear scale factor while providing a higher signal-to-noise ratio, i.e., lower jitter, than the prior art.
Applications for the radiolocation system include handwriting digitizing, wireless computer input devices such as a computer mouse (which can, for example in combination with a handwriting pen, control point, click, drag, pen writing line thickness and color, and paging functions), automated machinery control such as numerically controlled machines and digital surgery devices (scalpel location), virtual reality systems (including head position location, e.g., for pilots to control aircraft functions and for vehicle drivers), gambling machines (e.g., CRT-pen input), CRT/LCD based games with a pen input, and inventory tracking, e.g., for warehouses.